Well, perhaps the story could revolve around how their relationship changes with the new addition. They might struggle to balance their love for each other and their newfound parental responsibilities. For example, Damon's wild side might need to be tamed a bit more as he becomes a father, and Elena has to deal with the stress of being a mother in a world full of danger.
One possible plot could be that they face many challenges in raising the baby. Maybe there are threats from the supernatural world that target their child. Damon, with his protective nature, would be on high alert all the time, while Elena uses her compassion to raise the baby in a loving environment.
Another group could be the vampire hunters. If they find out about the baby, they would likely see it as an abomination and try to kill it. They could be a constant threat, forcing Damon and Elena to keep moving and hiding. They might also team up with other supernatural enemies of Damon and Elena to try and take down the baby.
Damon would likely become more protective and responsible. He might start to see the value in a more stable life. Elena would grow in her maternal instincts and become even more of a force to be reckoned with when it comes to protecting her family.
Another plot could be centered around the baby's upbringing in a world full of supernatural beings. Damon and Elena might struggle with how to raise the baby in a way that it can fit into both the human and supernatural worlds. They could face challenges like finding the right school or dealing with other vampires who might not be so accepting of a half - vampire baby. Maybe they would have to move to a new place to keep the baby safe.
Damon and Elena's relationship could become more centered around their family. In the past, their relationship was often about their own love and the supernatural drama around them. With a baby girl, they would focus on creating a stable home environment. They might have to sacrifice some of their own desires for the sake of their daughter. For example, Damon might have to give up some of his more dangerous vampire - related activities to be a better father figure. And Elena would have to balance her human life with being a mother to a half - vampire baby girl. This new focus on family would transform their relationship into a more mature and committed one.
Perhaps the baby girl gets kidnapped by an old enemy of Damon's. This enemy thinks that by taking the baby, they can get revenge on Damon. Elena then teams up with Damon's brother Stefan to rescue their daughter. Along the way, they discover secrets about Damon's past that he has been hiding from Elena, and this discovery could either strengthen or strain their relationship while they are desperate to get their baby girl back.
Stefan might be initially shocked but then be very supportive. He would probably offer to help in any way he can, like babysitting or giving advice based on his own experiences. Bonnie would be excited and use her magic to create some special things for the baby, like a protective charm.
I think Stefan can be a really liked character in these fanfictions even though he's in a bit of a love triangle situation. His complex relationship with Damon and his past with Elena make him an interesting addition. He might be shown as still caring for Elena but also coming to terms with her love for Damon. Also, Alaric is a character that many fans like. He can act as a mentor or a voice of reason in the fanfiction, and his relationship with Damon and Elena can be explored in different ways.